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Beans are members of the Fabaceae, or Leguminosae, family - an incredibly diverse group containing trees, shrubs, vines, and forbs. Fabaceae is the third largest plant family, after Poaceae (the grasses), and Orchidaceae. It includes crops such as green and dry beans, soybeans, peanuts and peas as well as forages such as clover, alfalfa and the vetches, making this family the second most important (after Poaceae) in terms of human nutrition. Although several genera, species, and varieties of beans are important food or feed crops throughout the world, the species Phaseolus vulgaris includes many of the human eating favorites. These include common bean, caraota, feijao, french bean, kidney bean, haricot bean, field bean, poroto, snap bean, string bean, frijol, and wax bean. One different food species is the tepary bean, P. acutifolius, native to the Southwest U.S. and Mexico, a heat tolerant and drought resistant plant, that was an important part of the Native American diet. |
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The center of origin of beans is obscure, with some experts citing the Mediterranean area, and others claiming South America. Bean fossils have been found in Egyptian tombs, American caves, and Asian prehistoric sites. The beans used for human food are separated into two broad agronomical categories, green and dry. All beans are annuals. The plants produce pods which contain the seeds. In green, (or snap or string beans) the pods are harvested before ripening, and both the pods and the seeds are eaten. In dry (or field) beans, the pods and seeds are allowed to ripen and dry, then threshed, and only the seeds are eaten. |
| The Fabaceae family is associated with Rhizobium root nodule bacteria in a nitrogen-fixing symbiotic relationship. Bacteria of the genus Rhizobium induce nitrogen-fixing nodules on the roots of legume, thus reducing the requirements for nitrogen fertilizer for these crops. It seems that, similar to mitochondria and Agrobacteria, Rhizobia began existence as a plant pathogen, but one which became useful to the host plant. The bacteria work in a nodule ("little knot"), an enlargement of the root cells. Infection by Rhizobia bacteria is somewhat similar to an infection in the body. The bacteria enter through a susceptible location, travel inward, establish themselves, and multiply. Somehow the plant recognizes the particular beneficial bacteria and doesn't try to destroy it. Inside the nodule, an enzyme called nitrogenase drives the nitrogen-fixation reaction. Nitrogen is used in the plant to make amino acids, the building blocks of proteins. The bacteria convert nitrogen into a chemical form that the plant can use and the plant supplies the bacteria with sugar and other nutrients. While the majority of fixed nitrogen is removed with the crop, some residual nitrogen is available in the soil for the next crop. |
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Links to other sites with information about beans:
| Crop
Plant Resources | | Outreach
and Educational Resources | | Molecular
Biology Program |
Please send comments and suggestions to: moconnel@nmsu.edu
Last Updated: 8/05
